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A friend wants 240 and 120 in his shed. The beaker panel is in the garage about 40 feet away. It was suggested to run 4 individual wires--two for the 240 and one for ground and one for neutral.

What breaker and/or breakers should be used and how would they be connected? I know neutral goes to the breaker bar and the same with ground and the grounding rod outside. But how do I "break off" one leg for 120 and have it properly connected to a breaker? Thanks.

  • I think the electrician was probably going to set a sub panel in the shed. – Tyson Jun 16 '16 at 20:22
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    Regardless of your prior experience with x-ray machines, my strong feeling is that you leave this job for someone who knows what they are doing. – Speedy Petey Jun 16 '16 at 20:47
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Since this would be a sub-panel install, ground and neutral are NOT connected - they are isolated. There is but one bond between them per code, and that one already exists in the system that the power is coming from.

I'd tend to agree with S.P. that your prior experience is either too long ago or too unrelated to what you are doing for it to seem a great idea for you to dive into this, unless you'd care to prove me (us) wrong by actually doing some research and modifying your question. Going into it half-cocked and not actually understanding things like why there are 4 wires and the neutral (grounded) and ground (grounding) conductors are isolated seems a recipe for disaster I'd rather not encourage.

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You'll need a double pole breaker. You'll connect the neutral to the neutral bar, the grounding conductor to the grounding bar, one ungrounded (hot) conductor to one terminal of the breaker, and the other ungrounded (hot) conductor to the other breaker terminal.

At the panel in the shed, you'll connect the neutral to the neutral bar, the grounding conductor to the grounding bar, one ungrounded (hot) conductor to one lug of the panel, and the other ungrounded (hot) conductor to the other panel lug.

You'll disconnect the bonding strap between the grounding and neutral bars in the shed panel, and keep all grounding and neutral conductors separate.

You'll have to install a grounding electrode system at the shed, and connect the grounding bar in the panel to it.

Aside from that, you'll have to determine what size feeder conductors to use. Which means doing a load calculation, and figuring for voltage drop on the feeder and any other adjustments. Then you'll select a breaker for the main panel, so that it's sized properly to protect the feeder conductors.

You'll have to have a means of disconnect at the shed, so that you can easily shut power off.


While this type of project can be done by a DIYer, it's often best to leave it to a professional. Or at least have a professional involved during the planning process, and again for a final inspection.

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I agree that more knowledge is a good thing, and more knowledge is obtainable - hit one of those big-box home improvement stores' bookshelf section (they only thing they're good for) and paw through "how to wire a house" books til you find one that you vibe with. Or hit a library but those books may not reflect current code. This is learnable.

Now, depending on what you're doing, you have some options.

First, neutral must absolutely be separated from ground in a sub-panel (and everywhere but the main panel). Bring over one wire for each. Keep them rigorously separated in the sub-panel. Other answers here explain in depth why. Neutral is not ground! As such, the sub-panel will have 2 bars - a neutral bar and a ground bar, and you will remove the strap bonding them together.

Most likely you'll be installing a sub-panel, which means 4 wires, with a breaker in the main panel - its size is defined by the size of those wires, which will in turn be based on common-sense thinking about what loads you plan to have in the garage.

The breakers in the sub-panel will be whatever fits the wiring and loads you attach to them. For instance all outlet loads will be 15A or 20A breakers depending on whether you wired them with 14AWG or 12AWG wire.

Make a 2 column chart of the amperage all the loads you expect to have out there. 240V loads go in both columns. 120V loads go in one column or the other. Total up the amperage in each column. Take the largest of the two, add 25% or a larger margin for expansion if you prefe. Post your results as a question here and let us check it.

Note that it helps to distribute 120V loads on both sides evenly. For instance if you have an 8 amp air conditioner and a 7 amp dust collector, put them on opposite sides so your largest number is smaller. The opposite sides on the spreadsheet will eventually be opposite poles on the panel.

Put at least some lighting on its own dedicated circuit. It is preferable that a saw tripping a breaker should not plunge you into darkness.


If you want to reduce wire count or avoid having a sub-panel, that's possible:

If you mainly wanted 240V 20A over there, and only ever use 120V occasionally, you could wire a single 240V circuit powering outlets and lights (yes they have 240V lights, most new fluorescent and LED shop lighting is multi-voltage 120-277V). For temporarily connected 120V loads, you can use a common step-up/down transformer. This setup would only require 3 wires (2 hot 1 ground no neutral) and the breaker would be back in your main panel. You would use NEMA 6-20 outlets which are good for 4800 watts.

You could add a permanent 120V circuit to the above with 2 more wires (hot and neutral, borrow the other circuit's ground). That's 5 total wires with redundant hots. The hots or neutrals cannot be combined or borrowed this way, that is what sub-panels are for.

If you really, really want to have a "sub"-panel with 3 wires, there's a way, but it'll involve a large transformer and making the garage its own main panel. It's also pretty advanced. Not something I recommend for your first big project.

  • Yeah, getting involved with the separately derived supply rules in the Code is not what I'd recommend for someone's first rodeo! – ThreePhaseEel Jun 17 '16 at 1:44

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